Conventionally, for example, a semiconductor device is proposed as a switching device used for an inverter and the like such that an IGBT region and a diode region are commonly disposed on a semiconductor substrate, an IGBT element is formed in the TGBT region, and a diode element is formed in the diode region (See for example, Patent Literature No. 1).
Specifically, in the semiconductor device, a base layer is formed in a surface portion of the semiconductor substrate, which constitutes a drift layer having a N− conductive type, and multiple trenches are formed to penetrate the base layer. A gate insulation film is formed in each trench so as to cover a sidewall of the trench, and a gate electrode is formed on the gate insulation film.
On a backside of the semiconductor substrate, a cathode layer having a N conductive type and a collector layer having a P conductive type are formed. AN emitter region having the N conductive type is formed in a portion of the base layer disposed above the collector layer. Further, an upper electrode, which is electrically connected to the base layer and the emitter region, is formed on a surface side of the semiconductor substrate. On the backside of the semiconductor substrate, a lower electrode, which is electrically connected to the collector layer and the cathode layer, is formed. A region in which the collector layer is formed on the backside of the semiconductor substrate is defined as the IGBT region. A region in which the cathode layer is formed on the backside of the semiconductor substrate is defined as the diode region.
Here, the trench is formed in each of the IGBT region and the diode region. The gate electrode formed in the IGBT region and the gate electrode formed in the diode region are connected to a common gate pad so that a common voltage is applied to each other.
In the above semiconductor device, in the IGBT element, when a voltage lower than the lower electrode is applied to the upper electrode, and a turn-on voltage is applied to the gate electrode, an inversion layer (i.e., a channel) having the N conductive type is formed in a portion of the base layer contacting the trench. An electron is supplied to the drift layer via the inversion layer from the emitter region, and a hole is supplied from the collector layer to the drift layer. Thus, a resistance of the drift layer is reduced due to conductivity modulation, and it becomes to an on-state. Here, the turn-on voltage is a voltage for increasing the voltage Vge between the gate and the emitter to be higher than the threshold voltage Vth of the MOS gate.
In the diode element, a voltage higher than the lower electrode is applied to the upper electrode. When the voltage between the upper electrode and the lower electrode becomes higher than a forward voltage, it becomes to the on-state. At this time, the electron in the drift layer is discharged to the upper electrode via the base layer; and the hole is injected into the drift layer from the base layer when the electron passes through the base layer.
However, for example, in a case where an inverter circuit is formed using multiple semiconductor devices, the turn-on voltage may be applied to the gate electrode even when the diode element is in the on-state. When the diode element is in the on-state, and the turn-on voltage is applied to the gate electrode, the inversion layer having the N conductive type is formed in a portion of the base layer contacting the trench. In this case, in the diode region, the electron in the drift layer is discharged to the upper electrode via the inversion layer, and therefore, the electron does not pass through the base layer. Accordingly, the hole is not supplied to the drift layer from the base layer, and the conduction loss of the diode element becomes large.
Here, in order to solve the above difficulties, it is considered that no trench is formed in the diode region. However, when no trench is formed in the diode region, an electric field concentration occurs near the trench in the IGBT region formed on a diode region side. Thus, a new difficulty arises such that breakdown voltage is reduced.